A typical laser apparatus of the type contemplated by the present invention is a triaxial orthogonal CO.sub.2 laser wherein the axis of laser light, the path of d.c. glow discharge and the direction of gas flow intersect at right angles. A logitudinal section of a conventional triaxial orthogonal laser is shown in FIG. 1. A cross section taken on the line II--II of FIG. 1 is shown in FIG. 2, wherein like components are indentified by like numerals. In FIG. 1, the numeral 10 indicates an anode, 12 shows a plurality of cathodes, 14 is a cathode substrate made of an insulator material, 18 designates stabilizing resistors, 16 is a high-voltage d.c. power source, 20 is a discharge exciting area, 22 is a total reflection mirror, and 24 is a partial reflection mirror having a proper reflectance. The laser of FIG. 1 is operated as follows. A laser gas made of a mixture of CO.sub.2, N.sub.2 and He is caused to flow between anode 10 and cathode 12 at a rate of several tens of meters per second in the direction indicated by arrow B. A voltage from power source 16 produces a discharge between the electrodes, and current flows through stabilizing resistor 18. The discharge will be a nondisruptive glow discharge without changing to an arc discharge. As a result of the glow discharge, exciting area 20 causes a population inversion between specific ocsillation levels of the CO.sub.2 molecule in the laser gas. Laser oscillation takes place between total reflection mirror 22 and partial reflection mirror 24, and a laser beam is emitted from partial reflection mirror 24. Therefore, total reflection mirror 22 and partial reflection mirror 24 make up a laser oscillator. The laser output is increased as the discharge power is increased, and with a given number of cathodes 12 shown in FIG. 1, the increase in discharge power is equivalent to the increase in discharge density. To reduce the size and cost of the apparatus, the discharge density is desirably increased, but if the discharge power exceeds a certain level, a hot spot occurs in a certain part of the discharging area, and the glow discharge changes to an arc discharge in spite of stabilizing resistors 18. With an arc discharge, a laser output is no longer produced and the laser gas is considerably deteriorated. Therefore, as a compromise between the requirements of small size and high reliability, conventional laser oscillators are designed so that maximum output is produced just before the glow discharge changes to an arc discharge.
When laser light is used on a workpiece made of a material having high thermal condectivity, such as metal, the thermal energy is dissipated through the workpiece. A pulsed laser having the same average output as a continuous laser of uniform output will have peaks at intervals providing a larger working energy and achieves a higher working precision than the laser output from the continuous laser. Therefore, in the conventional laser of the type shown in FIG. 1, the output from d.c. high-voltage source 16 is converted into a pulsed output as shown in FIG. 3(a) and produces a pulsed laser output as shown in FIG. 3(b).
With such a pulsed laser oscillator, the peak laser output obtained is not greater than the maximum output of time-independent laser oscillation (continuous oscillation), and to obtain an average output equal to that achieved by continuous oscillation, a large oscillator using an increased number of cathodes 12 is necessary. For instance, if the duty factor is 50% as shown in FIG. 3(a), to obtain an average laser output equal to that achieved by time-independent (continuous) oscillation, the size of the oscillator and power capacity must be doubled. Additionally, where the laser oscillation is caused only by glow discharge, as in FIG. 1, if the rise time of a voltage pulse is too short, insufficient heat diffusion in the interior of the glow discharge will cause thermal non-equilibrium, and this will produce an arc even if the current flowing is below the arc producing level. To prevent the latter from occurring, the rise time of current applied to the conventional laser oscillator cannot be shorter than a certain period, and the resulting pulsed output has a maximum frequency of only about several tens of hertz.
But as is well known, working with laser light requires higher pulse frequencies for achieving higher working precision and working speed, and thus, the development of a more compact laser apparatus that achieves higher frequencies and which is also easy to operate has been desired. For instance, to cut a metal or nonmetallic plate with a laser beam, the laser output must be switched on and off or slope-controlled in synchronism with the travel of the work. Conventionally, this is achieved by controlling the current flowing through d.c. high-voltage power source 16, but it has been difficult to secure reliable control of the large power necessary for cutting. For example, discharge power as great as 5 to 10 kw is necessary to cut a metal plate as thick as several millimeters. A vacuum switch or ignitron is usually employed to control such large power, but these switching devices are short-lived or require much skill and time for maintenance. Furthermore, d.c. high-voltage power source 16 usually includes a rectifier circuit and a smoothing circuit, and the time constant of the smoothing circuit is a limiting factor on the control speed.